The present invention relates to a sintered target for depositing an underlayer of magnetic recording media for use in a magnetic disk drive, etc., which comprises a sintered body of a intermetallic compound with a B2-ordered lattice structure, a production method of the sintered target, and a magnetic recording medium with a B2-structured underlayer formed by sputtering the sintered target.
Many improvements have been made on a Co-based magnetic layer of magnetic recording media to achieve a high density magnetic recording. For example, an underlayer is disposed under the magnetic layer to allow the magnetic layer to grow epitaxially on the underlayer. To ensure a good epitaxial growth of the magnetic layer, several considerations and improvements have been made on the underlayer with respect to the lattice constant, the crystalline orientation, the grain size and the uniformity of the film structure. As the material for the underlayer, pure Cr and Cr alloy have been widely used in the art because of a good lattice match between the Co-based magnetic layer and the Cr-based underlayer.
As an underlayer having a lattice constant identical to that of a Cr-based film, a thin film of an intermetallic compound with a B2-ordered lattice structure has been proposed in EP 0704839 Al, U.S. Pat. No. 5,693,426, IEEE Trans. Magn., vol. 30, 3951 (1994), IEEE Trans. Magn., vol. 31, 2728 (1995), etc.
The B2-ordered lattice structure may be referred to as CsCl structure, and includes lattice positions (0, 0, 0) and (1/2, 1/2, 1/2). In an NiAl intermetallic compound having a typical B2-ordered lattice structure, when the atomic ratio, Ni/Al, is 1, Ni atoms occupy the positions (0, 0, 0) and Al atoms occupy the positions (1/2, 1/2, 1/2). When Ni/Al&gt;1, Ni atoms occupy the positions (0, 0, 0) and Ni atoms and Al atoms occupy the positions (1/2, 1/2, 1/2). When Ni/Al&lt;1, Ni atoms and voids occupy the positions (0, 0, 0) and Al atoms occupy the positions (1/2, 1/2, 1/2). The lattice constant of NiAl is 0.2887 nm when Ni/Al=1, almost identical to that of pure Cr 0.2884 nm.
The above documents propose to deposit a thin film of the intermetallic compound with the B2-ordered lattice structure, in place of a Cr underlayer, as the underlayer on which the magnetic layer is directly deposited, and alternatively to deposit a thin film of the intermetallic compound with the B2-ordered lattice structure under a pure Cr intermediate layer. It is reported in the documents that the B2-structured layer under the Cr intermediate layer reduces the grain size of the Cr intermediate layer and provides a magnetic recording medium with increased coercive force.
As a result of studies on the underlayer having the B2-ordered structure, the inventor has confirmed that the underlayer of the intermetallic compound with the B2-ordered lattice structure has a lattice constant close to that of the Co-based magnetic layer, and also has a crystalline orientation and a fine, uniform structure suitable as the underlayer of Co-based magnetic recording media.
The underlayer of the intermetallic compound with the B2-ordered lattice structure may be usually deposited by RF-sputtering a target as described in the above documents. However, the documents teach nothing about the detail of the target for forming the B2-structured thin film.
The inventor has attempted to produce a target for depositing a uniform film of the B2-ordered lattice intermetallic compound by a melting/casting method. However, the resultant cast target was too poor in bending strength and easily broken during the machining process and sputtering process. Further, relatively small chill crystals, coarse columnar crystals extending along the cooling direction and relatively small equiaxed crystals were formed in the cast target, and there was a larger variation in the grain size as shown in FIG. 3. The large variation in the grain size of the cast target causes the particle deposition and the nonuniform thickness of the thin film being deposited. If the thickness of the underlayer is uneven, a thin film to be deposited thereon becomes difficult to epitaxially grow, and as a result thereof, the characteristics of magnetic recording media are deteriorated.